Thermal imaging systems are often employed to detect fires, overheating machinery, planes, vehicles and people, and to control temperature sensitive industrial processes. Thermal imaging systems generally operate by detecting the differences in thermal radiance of various objects in a scene and by displaying the differences as a visual image of the scene. Thermal imaging systems must effectively deal with the relatively small difference in thermal radiance between the objects in the scene as compared to the total thermal radiance emitted by the scene.
The basic components of a thermal imaging system generally include: optics for collecting and focusing thermal radiation from a scene; a thermal detector having a plurality of thermal sensors for converting thermal radiation to an electrical signal; and electronics for amplifying and processing the electrical signal into a visual display or for storage in an appropriate medium.
The thermal sensors of a thermal imaging system may be disposed in a focal plane array. The focal plane array and its associated thermal sensors are often coupled to an integrated circuit substrate with a corresponding array of contact pads and a thermal isolation structure disposed between the focal plane array and the integrated circuit substrate. The thermal sensors define the respective picture elements or pixels of the resulting thermal image.
One type of thermal sensor includes a thermal sensitive element formed from pyroelectric material which exhibits a state of electrical polarization and/or change in dielectric constant dependent upon temperature changes of the pyroelectric material in response to incident infrared radiation. A pair of thin film electrodes are generally disposed on opposite sides of the pyroelectric material to act as capacitive plates. In this arrangement, the pyroelectric material acts as a dielectric, or insulator, disposed between the capacitive plates. Accordingly, the electrodes are operable to measure the charge generated by the pyroelectric material in response to changes in temperature. As previously discussed, the charge, or electrical signal, may be amplified and processed into a visual display.
A chopper is often included in a thermal imaging system to dynamically couple the detector to the scene. The chopper rotates a series of windows through the thermal radiance from the scene. These windows alternately interrupt the thermal radiance from the scene to produce a constant background radiance. During the time period that the thermal radiance is interrupted, a reference signal is recorded from the focal plane array. During the time period that the thermal radiance is uninterrupted, a scene signal is recorded from the focal plane array. The electronic processing portion of the thermal imaging system will subtract the reference signal from the scene signal to produce a signal with a minimum of background bias.
Choppers can be either opaque or diffusing. During the reference signal phase of operation, the opaque chopper completely interrupts the thermal radiation from the scene to the focal plane array. Whereas, during the scene signal phase of operation, the thermal radiation is uninterrupted to the focal plane array. One disadvantage with the opaque chopper is that the difference in the reference signal from the scene signal includes a large DC signal component which corresponds to the total difference of the thermal radiance emitted by the scene relative to the opaque chopper. This forces both the focal plane array sensors and the electronic processing portion of the thermal imager to adjust to a wide thermal range during operation. In addition, the focal plane array sensors have a limited thermal dynamic range and the high DC component of the signal produced by the scene leaves less dynamic thermal range for actually representing the individual objects in the scene. A further problem with opaque choppers is that this large DC component of the signal gets multiplied by any gain non-uniformities in the detector, which then requires the electronic processing portion of the thermal imaging system to process the full DC component of the signal to compensate for these non-uniformities, which requires a higher digital resolution for an equivalent performance by a diffusing chopper system (in excess of 10 bits of digital processing).
Diffusing choppers have attenuated many of the problems observed with opaque choppers. Diffusing choppers differ from the opaque chopper in the amount of thermal radiation that they block. Instead of completely blocking thermal radiation from the scene during the reference signal phase of operation, a diffusing chopper uniformly averages or diffuses the thermal radiation from the scene across the focal plane array; thus, the term diffusing chopper.
A diffusing chopper system does not create a large DC signal component regardless of the scene condition. This dynamically couples the reference signal to the scene signal created by the detector. This is the most efficient and lowest cost signal processing architecture (as few as 8 bits of digital processing).
One problem with many diffusing choppers is that the chopper window utilized during the scene signal phase of operation is covered with the same transparent material that covers the chopper window during the reference signal phase of operation. The difference between scene signal and the reference signal chopper window is that the chopper window used during the reference signal includes a diffusing pattern. The materials used to cover the chopper windows are selected to be highly transparent to the thermal radiation from the scene, but they are not fully transparent. This creates a problem because the thermal radiation from the scene reaching the thermal sensors is attenuated during the scene signal phase of operation.
An additional problem with many diffusing choppers is that a residual signal artifact is created when the scene signal is subtracted from the reference signal as a result of fixed pattern noise that is common to both signals. Due to imperfect averaging of the thermal radiation from the scene during the reference signal phase, a "halo" artifact created. This artifact is undesirable in a thermal image.
A further disadvantage of many diffusing choppers is the expense of the high thermal transparency material, e.g. silicon or germanium, and the cost associated with a surface treatment of the material, e.g. binary lensets as described in U.S. patent application Ser. No. 60/024,048 entitled "Infrared Chopper Using Binary Diffractive Optics."
Additionally, many choppers often generate a background that is not uniform or that greatly differs from the thermal energy of the scene. A background signal that is not uniform may prevent adequate normalization of the thermal sensors. A background signal that substantially differs from the scene signal may expose the thermal sensors to a large dynamic range that degrades the quality of the image obtained from the scene.